Civil engineering and industrial coating industries require economical, environmentally safe resin systems which cure at ambient temperature and give good elastomeric properties without compromising the moisture and chemical resistance. Concrete structures, for example, require coatings which are able to withstand impact as well as cover shrinkage-induced cracks which may be present therein.
Aromatic polyepoxide resins, particularly those based upon the use of polyglycidyl ethers of bisphenol A, have widely been used for preparing cast resins and coatings for civil engineering applications. Some of the reasons for success of epoxy resins in these applications due to their moisture and chemical resistance as well as their ability to bond to a variety of substrates. When the polyepoxide resins are cured with polyamides, the resins, although resistant to chemicals, generally lack elasticity for a multitude of applications and when cured with amines, the polyepoxide resins lose their flexibility and impact strength.
Historically, flexibility in an epoxy resin formulation has been obtained through the use of coal tar or similar additives. Flexibility has also been imparted to an ambient cured epoxy by making gross changes in the cured epoxy structure. These changes typically have been accomplished by the following methods: (1) modifying the aromatic character to a more aliphatic character; (2) reducing the crosslink density by using a curative with lower functionality or using a high excess of curative; and (3) adding long chain modifiers or flexibilizers in the form of resin, curative, or reactive additive. To achieve flexibilization of the epoxy resin pursuant to method 3, often flexibilizers in excess of sixty percent by weight of the formulation may be required or plasticizers such as nonylphenol or benzyl alcohol may be used to reduce the concentration of flexibilizer. The use of plasticizers leads, of course, to the presence of fugitive materials.
Representative patents which describe polyepoxide resins and the approach to enhancing the physical properties of the polyepoxide resins are as follows:
U.S. Pat. No. 3,316,323 discloses the preparation of bisphenol A type epoxy molded articles having improved flexibility and reduced water absorption which incorporate an outer softening agent such as phthalic acid ester or an adipic acid ester. Inner softening of the epoxy resin is achieved by incorporating the addition of aliphatic polyetheralcoholdiglycidyl ethers, e.g., polyethyleneglycoldiglycidyl ether where the polyethyleneglycoldiglycidyl ether carbon atoms in the .alpha. and .alpha.' positions are substituted with an alkyl, an alkylene, aralkyl or aryl side chain. By incorporating the hydrocarbon substituent on the .alpha. and .alpha.' carbon atom, reduced water absorption values are obtained.
U.S. Pat. No. 3,567,677 discloses the preparation of epoxide pottable encapsulated electrical connectors having high flexibility. The compositions comprise diglycidyl ethers of bisphenol A, diepoxides of a polyglycol, and at least one amine curing agent. Formulations include diglycidyl ethers of bisphenol A, diepoxides of polyglycols and amine curatives, including amine curatives adducted with phenylglycidyl ether in a 1:1 molar ratio.
U.S. Pat. No. 4,608,313 discloses epoxy resins crosslinked with polyisocyanates which are resistant to chipping by stone. Aliphatic diepoxides and aromatic diepoxides are utilized in the stone resistant epoxy formulations with the aliphatic diepoxides being glycidyl ethers of polyoxyalkylene glycols generated by reacting an aliphatic initiator with propylene oxide or butylene oxide.
U.S. Pat. No. 4,552,814 discloses chip-resistant epoxy resins particularly adapted for protective coatings on automobiles. The formulations incorporate polyglycidyl ethers of compounds having aliphatic hydroxyl groups such as glycidyl ethers of ethylene and propylene glycol as well as the glycidyl ethers of polyoxypropylene and polyoxyethylene glycol.
U.S. Pat. No. 3,299,169 discloses epoxy resins having improved flexibility while retaining good tensile strength comprising glycidyl ethers of bisphenol A, a diglycidyl ether a polyglycol and a polyoxyalkylenediamine curing agent.
U.S. Pat. No. 4,277,390 discloses high molecular weight polyepoxides which have high impact strength, good heat resistance, chemical stability, as well as high elasticity and low brittleness. The patentees' combine (polypropylene glycol)diurethanediepoxide with the polyglycidyl ethers of bisphenol A and curing with an amine curing agent, e.g., methylene dianiline.
U.S. Pat. No. 2,528,932 discloses polyepoxide resins having enhanced spreadability or fluid characteristics when utilized as an adhesive. To enhance the fluidizing property of the polyepoxide resin, a normally liquid, substantially nonvolatile mono-epoxy compound is incorporated into the formulation. Examples include glycidyl ethers of polyhydric phenols as the resinifying constituent and representative mono-epoxy compounds include glycidylphenyl ether, styrene oxide and .beta.-methyl-glycidol.